1. Field of the Invention
This invention relates to an electrode structure having a current collector of a lithium secondary battery, electric double-layer capacitor or the like, and a lithium secondary battery and electric double-layer capacitor using the electrode.
2. Background Art
Secondary batteries for use in notebook size personal computers, video cameras and the like are required to have a high energy density and a satisfactory charge/discharge cycle life. The secondary batteries which have been used heretofore include lead acid batteries, nickel-cadmium batteries, and nickel-hydrogen batteries. Lithium secondary batteries were proposed and have been used in practice as a secondary battery having a higher energy density.
In the prior art, liquids are used as the electrolyte of such secondary batteries. If the electrolyte is made solid, it is attractive as batteries of the next generation type because solid electrolytes can prevent liquid leakage and enables sheet structuring. In particular, if lithium ion secondary batteries which now enjoy rapid widespread use in notebook size personal computers and the like can be structured into a sheet form or multilayer compact size, they are expected to find a wider range of application.
In order that such solid electrolytes be used, it is proposed to construct them as ceramic materials, polymeric materials or composite materials. Among others, gel electrolytes which are plasticized using a polymeric electrolyte and an electrolytic solution are considered promising in the development of electrolytes because they have both the high conductivity of liquids and the plasticity of polymers.
An example of utilizing a gel electrolyte in a battery is already disclosed in U.S. Pat. No. 3,985,574. This patent includes examples in which a polyacetal which has been gelled by incorporating therein a supporting electrolyte such as ammonium perchlorate and a solvent such as propylene carbonate is used as a separator or positive electrode. A battery is constructed using lithium as the negative electrode. The positive electrode is prepared by bonding a honeycomb grid to a metal such as silver under heat and pressure and applying the gel thereto. Further, U.S. Pat. No. 5,296,318 discloses a practical system.
Specifically, U.S. Pat. No. 5,296,318 discloses that a copolymer of poly(vinylidene fluoride) with hexafluoropropylene is used as polymeric matrices of a positive electrode, negative electrode and separator of a lithium intercalation battery. The key feature of this disclosure is the use of the specific copolymer. The reason why characteristics can be improved is believed to be that since not only a binder alone is added to the electrode, but a gel electrolyte composition is also mixed during formation of the electrode, the electrode well matches with the electrolyte so that the internal resistance can be reduced. That is, unlike the solution system, an electrode composition inherent to the gel electrolyte system (referred to as gel electrode composition, hereinafter) is constructed.
Further, U.S. Pat. No. 5,470,357 discloses an electrode structure wherein a metal grid of aluminum or copper is used as the current collector. The method of making electrodes involves forming a film of a gel electrode composition and then bonding a metal grid thereto by the application of heat and pressure. The electrode composition cannot be applied to the grid during formation of the electrode because when a practical slurry is used, it cannot be fully held by the grid. Also, the results of our study showed that the use of a copolymer of poly(vinylidene fluoride) with hexafluoropropylene as the gel electrolyte was impractical because adhesion between the electrode and the collector metal was weak. Accordingly, it is believed that the manufacturing process described in Examples of U.S. Pat. No. 5,470,357 is used in practice. More particularly, it is necessary to previously coat the collector with a paint in which a conductive aid is mixed with a polymeric matrix component in a gel electrolyte, a paste in which a conductive material is added to a hot-melt paint as shown in the above-referred patent, or a conductive paint for use in electrolytic capacitors. Usually, the electrode (film+collector) is prepared by bonding the thus obtained grid to the gel electrode composition film under heat and pressure. It is thus apparent that a special step is necessary during formation of the electrode, leading to a reduced mass productivity and an increased cost.